拆分实体数据模型的属性上的数据注释[必需]属性

Question

I'm using the Table Splitting feature of the Entity Framework to split my Entity Data Model as follows:

+--------+    +--------------+
|  News  |    |  NewsImages  |
+--------+    +--------------+
| NewsID |    | NewsID       |
| Text   |    | Image        |
+--------+    +--------------+

Each News entity contains a navigation property called NewsImage to reference the corresponding image.

---

I'm using DataAnnotations to validate my Model. I put the [Required] attribute on the Text property of the News class:

[MetadataType(typeof(NewsValidation))]
public partial class News
{
    /* ... */
}

public class NewsValidation
{
    [Required]
    public string Text { get; set; }
}

---

Here is the server-side code I use to obtain the image data:

[HttpPost]
public ActionResult Create(News news)
{
    if (ModelState.IsValid)
    {
        UpdateNewsImage(news);
        _newsRepository.Add(news);
        _newsRepository.SaveChanges();

        return RedirectToAction("Index");
    }
}

private void UpdateNewsImage(News news)
{
    byte[] newsImage = GetNewsImage();
    news.NewsImage = new NewsImage { Image = newsImage };
}

private byte[] GetNewsImage()
{
    foreach (string upload in Request.Files)
    {
        HttpPostedFileBase uploadedFile = Request.Files[upload];

        if (!uploadedFile.HasFile())
        {
            break;
        }

        Stream fileStream = uploadedFile.InputStream;
        int fileLength = uploadedFile.ContentLength;
        byte[] fileData = new byte[fileLength];
        fileStream.Read(fileData, 0, fileLength);

        return fileData;
    }

    return null;
}

After calling the UpdateNewsImage(news) method, the entity news is populated correctly with the corresponding image data but the ModelState.IsValid property is still false ; debugging ModelState.Values results in one error: "The NewsImage field is required." .

---

How can I put the [Required] attribute (or some other mechanism enforcing an image for each News entity) on the NewsImage property?

Answer 1

Why not putting a [Required] attribute on NewsImage property inside your validation class? This will make it required that a News entity instance has a corresponding NewsImage entity instance as well.

When putting RequiredAttribute on reference (as in non-string) type properties it only checks that property is not null. Let me back this up by the RequiredAttribute.IsValid() method:

public override bool IsValid(object value)
{
    if (value == null)
    {
        return false;
    }
    string str = value as string;
    if (str != null)
    {
        return (str.Trim().Length != 0);
    }
    return true;
}

If you model state is invalid it means that your NewsImage is null. Maybe you're missing something obvious here and some other property invalidates your model state.

Asp.net MVC file data binding

I get it. It seems that you think that model state will be validated each time you do something to your model. That is of course not the case. Asp.net MVC automatically validates your action parameters for you before executing the action. So when your model state is invalid when you're in the body of your action method, it will stay that way no matter what you do to your model objects. Unless of course you manually manipulate model state. In your case when you add the image to your News still doesn't change model state (even though your object becomes valid).

As much as I understand it you have a problem with types. Asp.net MVC default model binder is able to automatically bind posted file streams to HttpPostedFileBase variables. Your NewsImage.Image property is of type byte[] hence it doesn't get auto-bound.

The problem is that you're using data model entities in your web application as application/view model entities so you can't just change NewsImage.Image type because it's part of your EF data model.

To get this thing to work I suppose the best/easiest way would be to write a separate view model entity class (don't confuse it to EF data model) with correct property types and add it a public method that converts it to data model News entity.

namespace WebProject.ViewModels
{
    public class News
    {
        public int Id { get; set; } // not used when creating new entries but used with editing/deleting hens not being required

        [Required]
        public string Text { get; set; }

        [Required]
        public HttpPostedFileBase Image { get; set; }

        public Data.News ToData()
        {
            return new Data.News {
                Id = this.Id,
                Text = this.Text,
                NewsImage = new Data.NewsImage {
                    Id = this.Id,
                    Image = // convert to byte[]
                }
            }
        }
    }
}

Your view model object will therefore be correctly model bound and validated as expected. The good thing is that your code will get simplified due to this change as well. Use ToData() method when you need to get data entity instance from this view model object instance. You could of course provide the oposite as well, by providing a constructor that takes the data model entity object instance and populate view model's properties.

If you use a separate database project where you keep your EF data model, I suggest you put your view model class strictly in the web application project (or any other application model project) because that's where it's used.